Who retreated to Washington leaving the confederates free to attack the north?

General Robert E. Lee and his Confederate Army of Northern Virginia advanced into the North after the Union forces, led by General George B. McClellan, retreated to Washington, D.C. following the Battle of Antietam. This retreat left the Confederates in a position to threaten Northern cities, leading to further incursions into Union territory. The situation culminated in the significant Battle of Gettysburg in 1863.